It is known to produce an oxygen rich product gas from pressure swing adsorption (PSA) plants. However, the product gas so produced is often at low pressure, that is, either close to atmospheric pressure or about 1 or 2 bar. If oxygen PSA plants are operated with a feed gas pressure which is higher than usual in order to provide the product gas at a higher pressure, then the cycle becomes progressively more inefficient in the use of power since air contains only 21% oxygen of which about half is trapped in the PSA plant so that consequently up to 90% of the initial compression energy can be wasted.
One alternative is to use mechanical compression for the oxygen product gas. However, the mechanical compression of oxygen is a known hazard and machines which meet the various safety requirements are very expensive.